The invention relates to a combined-cycle power plant which includes a gas turbine operable with both gas and oil as fuel and which has a waste-heat steam generator downstream of the gas turbine on the flue-gas side for generating steam for an associated steam turbine having at least one high-pressure stage.
In a combined-cycle power plant, the heat contained in the expanded working medium from the gas turbine is utilized in order to generate steam for the steam turbine. Heat transmission takes place in a waste-heat steam generator which is downstream of the gas turbine and in which heating surfaces in the form of tubes or tube bundles are disposed. Those heating surfaces, in turn, are connected into a water/steam circuit of the steam turbine. The water/steam circuit includes one or more, for example two or three, pressure stages. Each pressure stage conventionally has a preheating heating surface (economizer), an evaporator heating surface and a superheater heating surface. Depending on the pressure conditions prevailing in the water/steam circuit of the steam turbine, a thermodynamic efficiency of about 50% or more is achieved through the use of a combined-cycle power plant of that type which is known, for example, from European Patent 0 148 973 B1.
The gas turbine of a combined-cycle power plant of that type may be constructed to operate with different kinds of fuel. However, the requirements placed on the waste-heat steam generator downstream of the gas turbine on the flue-gas side are different, depending on the type of fuel on which the construction is based. For example, gas used as a fuel for the gas turbine normally has high purity, so that flue gas flowing out of the gas turbine contains only small amounts of impurities.
In contrast thereto, if the fuel for the gas turbine is fuel oil, impurities in the flue gas flowing out of the gas turbine are to be expected. In that case, in particular, sulfur dioxide (SO2) or sulfur trioxide (SO3) may occur, which, after reacting with water in the form of sulfuric acid (H2SO4), may settle on the heating surfaces in the waste-heat steam generator and attack them. The requirements placed on the waste-heat steam generator when oil is used as a fuel for the gas turbine must therefore be different from those when gas is used as the fuel for the latter.
In particular, when oil is used as a fuel for the gas turbine, it is necessary to ensure that the heating surfaces connected into the water/steam circuit of the steam turbine and the line components inside the waste-heat steam generator are at a sufficiently high temperature, namely a temperature above the dew point of sulfuric acid. For that purpose, when the gas turbine operates with oil, the inlet temperature of the water or condensate flowing into the waste-heat steam generator is raised, as compared with the gas turbine operating with gas, and is set at about 120xc2x0 to 130xc2x0 C.
A combined-cycle power plant, in which fuel oil is only provided as a fuel for the gas turbine for a brief operating period, for example for 500 to 1,500 h/a, as a xe2x80x9cback-upxe2x80x9d to natural gas, is usually constructed and optimized primarily for the gas turbine to operate with natural gas. In order to ensure that, when the gas turbine operates with fuel oil, the condensate flowing into the waste-heat steam generator has a sufficiently high inlet temperature, the necessary heat may be extracted from the waste-heat steam generator itself in various ways.
One possibility is to bypass a conventionally provided condensate preheater completely or partially and to heat the condensate by the supply of low-pressure steam in a feed-water tank connected into the water/steam circuit. However, at low steam pressures, such a method necessitates a large-volume and possibly multi-stage heating steam system in the feed-water tank and, in the case of long heating-up periods, that may put at risk a deaeration function which normally takes place in the feed-water tank.
In order to ensure effective deaeration of the condensate, the condensate temperature in the feed-water tank must always be maintained in a temperature range of between 130xc2x0 and 160xc2x0 C., and the heating-up period of the condensate in the feed-water tank should be kept as short as possible. That may be carried out, for example, by preheating the condensate through an additional preheater heated through the use of steam.
In order to provide sufficient heat for that purpose, in the case of two-pressure or three-pressure plants it is often necessary to extract hot water from a high-pressure economizer of the waste-heat steam generator. However, the disadvantage thereof, particularly in the case of three-pressure plants, is that the delivery of a normally provided high-pressure feed pump may be influenced, and that the additional condensate preheater has to be constructed in a particularly inefficient way for the high pressure and large temperature differences.
Furthermore, in the case of fuel-oil operation, throttle losses of the feed pump or each feed pump occur adversely. Moreover, the extraction of hot water from the high-pressure economizer leads to a reduction in the high-pressure steam quantity due to a lowering of a so-called high-pressure approach temperature, thus in turn leading to a reduction in plant efficiency.
Another proven method is, when the gas turbine operates with oil, to assist the heating-up of the condensate in the feed-water tank or in the deaerator through the use of steam extracted from an intermediate superheater line. However, that method cannot be employed in the case of plants without a feed-water tank or without a deaerator.
The above-mentioned concepts of condensate preheating when oil is used as a fuel for the gas turbine are complicated in view of the components which are required as well as in view of the operating mode of the combined-cycle power plant. Moreover, plant efficiency is only limited when the gas turbine operates with oil.
It is accordingly an object of the invention to provide a combined-cycle power plant, which overcomes the hereinaforementioned disadvantages of the heretofore-known methods and devices of this general type and which achieve a particularly high plant efficiency at a low outlay in terms of apparatus and operation requirements, irrespective of a fuel used for a gas turbine.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for operating a combined-cycle power plant, which comprises utilizing heat contained in an expanded working medium of an associated gas turbine operable with both gas and oil as fuel, for generating steam for an associated steam turbine having at least one high-pressure stage; and after a change of the gas turbine from gas to oil operation, dividing feed water to be supplied to the at least one high-pressure stage into first and second partial streams, only one of the partial streams being preheated.
The invention proceeds from the concept that the condensate preheating which is additionally necessary when the gas turbine operates with oil is ensured by particularly simple measures and in a particularly simple way by transmitting the heat required for this purpose to the condensate not through the water/steam circuit, but instead through the flue gas from the gas turbine. In this case, the components, such as heat exchangers, mixing preheaters, steam reducing stations and/or corresponding pipelines, for example, which are necessary in the transmission of heat through the water/steam circuit, may be dispensed with. Instead, when a gas turbine operates with oil, the extraction of heat from the flue gas of the gas turbine is reduced at a suitable point, as compared with the operation of the gas turbine with gas, so that a sufficiently large amount of exhaust-gas heat is available for condensate preheating.
In this case, the feed-water preheating for the high-pressure stage of the steam turbine is provided by a suitable modification of the extraction of heat from the flue gas of the gas turbine. In a combined-cycle power plant constructed as a three-pressure plant, a corresponding modification of the feed-water preheating for the medium-pressure stage, which modification is dependent on the operating mode, may also be provided alternatively or additionally.
In accordance with another mode of the invention, after the change of the operation of the gas turbine from gas to oil, the operating pressure in a low-pressure stage of the steam turbine is increased. This ensures that the heat which remains in the flue gas due to the comparatively lower preheating of the feed water for the high-pressure stage, when the gas turbine operates with oil, is not transmitted to the water/steam circuit of the steam turbine through the low-pressure heating surfaces but, in actual fact, is carried further in the flue gas and is thus provided reliably for condensate preheating.
In this case, the operating pressure in the low-pressure stage may be set in such a way that steam production in the low-pressure stage comes to a stop. Expediently, however, the operating pressure in the low-pressure stage of the steam turbine is raised, for example to about 10 to 15 bar, in such a way that only some minimum steam production for maintaining the system functions still remains in the low-pressure stage.
In accordance with a further mode of the invention, in order to provide particularly high efficiency, even in a transitional phase after a change in the operating mode of the gas turbine, a branching ratio between the first and the second partial stream is advantageously set as a function of the temperature of the condensate to be supplied to the high-pressure stage. In this case, the temperature of the condensate flowing into the waste-heat steam generator may be monitored in a particularly favorable way.
With the objects of the invention in view there is also provided a combined-cycle power plant, comprising a steam turbine having at least one low-pressure stage and a high-pressure stage; a feed-water preheater associated with the high-pressure stage; a bypass line connected in parallel with the feed-water preheater; a gas turbine operable with both gas and oil as fuel and having a flue-gas side; and a waste-heat steam generator disposed downstream of the gas turbine on the flue-gas side for generating steam for the steam turbine.
In accordance with a concomitant feature of the invention, particularly favorable adaptation of feed-water preheating to the respective operating conditions is made possible by connecting into the bypass line a valve capable of being set as a function of the temperature of the condensate to be supplied to the low-pressure stage.
The advantages achieved through the use of the invention are, in particular, that a water inlet temperature into the waste-heat steam generator which is necessary when the gas turbine operates with oil and which is increased, as compared with the operation of the gas turbine with gas, is ensured by particularly simple measures. The complicated components that are conventionally provided for the additional condensate preheating that is necessary for this purpose, for transmitting heat from the water/steam circuit to the condensate, for example by the supply of low-pressure steam, may be dispensed with. Instead, sufficient heat transmission to the condensate is ensured due to the fact that the flue gas from the gas turbine still contains sufficient heat in the region of the condensate preheaters. The additional condensate-preheating heat which is necessary when the gas turbine operates with oil is therefore transmitted to the condensate directly through the flue gas. The outlay in terms of construction and operational requirements which is necessary for this purpose is particularly low.
Furthermore, components of the water/steam circuit, such as, for example, high-pressure feed-water pumps, may be given comparatively small dimensions, since they do not have to be constructed for a bypass mode, when the gas turbine operates with oil, with additional water extraction from the economizer. Moreover, depending on the structure of the low-pressure stage of the steam turbine and of the condensate pump, water inlet temperatures into the waste-heat steam generator of up to and above 130xc2x0 C. can be mastered. Virtually the entire fuel-oil spectrum for this purpose (back-up fuel) can therefore be covered, so that standardization is made possible.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a combined-cycle power plant, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.